I have found, whether writing historical fiction or non-fiction, the amount of research is about the same. You need a firm foundation on which to build the story, but the "mapping" process may be slightly different.
When I write historical fiction, I tend to start with a character and a problem in mind, sometimes I have an era for the setting, but sometimes the problem and character dictate that. An example is Breadline Blue, my new young adult novel from Little Creek Press. In the case of Blue, I started with the title which came from a Depression era song. So in a way, I suppose it did begin with an era. But it was the character that drove what came next.
In the case of my History Press books, where history, not the story, is paramount, I begin with a strong sense of time and place, then find the stories and characters to bring it to life. History Press originally contacted me because I am a storyteller. They wanted me to tell the history as story rather than divulging facts for the scholar, so I must blend traditional research with the art of fictional storytelling, while maintaining the truth and sharing legends at the same time. It's really quite a balancing act and loads of fun.
What happens when I begin a project like the History Press book I am working on now? I get folded into the work. I start out looking for one thing and find something else, that leads me in a whole different direction. The real struggle is to take control of it so I come back to where I wanted to be, which is writing a book. You can't afford to be waylaid by your own work until all you are doing is searching for information. It will throw you off schedule, off task, and into a great deal of confusion of facts. Therefore, my process has developed into somewhat of a virtual, and sometimes even physical, mapping, consisting of dates and names and places. Also, I try to write as I learn, rather than gathering all the research then writing. It does mean bouncing back to already written stories for fact checking and additions, but it progressively moves the book forward toward the finish line.
History has been a great interest for me, evidenced in the books I write. I know quite a bit, but I continue to learn with each new writing project.
Reminder to self: You MUST be focused, diligent, and dependable. Time and publishers wait for no man (or woman as the case might be.) In other words, I have seven to eight months before the rough draft of the new book needs to be on my editor's desk. Seems like a long time, doesn't it? It is not. And it MUST be on time.